Methods of making paper tissue, towel, and the like are well known, including various features such as Yankee drying, throughdrying, dry creping, wet creping and so forth. Conventional wet pressing processes have certain advantages over conventional through-air drying processes including: (1) lower energy costs associated with the mechanical removal of water rather than transpiration drying with hot air; and (2) higher production speeds which are more readily achieved with processes which utilize wet pressing to form a web. On the other hand, through-air drying processes have become the method of choice for new capital investment, particularly for the production of soft, bulky, premium quality tissue and towel products.
One method of making throughdried products is disclosed in U.S. Pat. No. 5,607,551 to Farrington, Jr. et al. wherein uncreped, throughdried products are described. According to the '551 patent, a stream of an aqueous suspension of papermaking fibers is deposited onto a forming fabric and partially dewatered to a consistency of about 10 percent. The wet web is then transferred to a transfer fabric travelling at a slower speed than the forming fabric in order to impart increased stretch into the web. The web is then transferred to a throughdrying fabric where it is dried to a final consistency of about 95 percent or greater.
There is disclosed in U.S. Pat. No. 5,510,002 to Hermans et al. various throughdried, creped products. There is taught in connection with FIG. 2, for example, a throughdried/wet-pressed method of making creped tissue wherein an aqueous suspension of papermaking fibers is deposited onto a forming fabric, dewatered in a press nip between a pair of felts, then wet-strained onto a through-air drying fabric for subsequent through-air drying. The throughdried web is adhered to a Yankee dryer, further dried, and creped to yield the final product.
Throughdried, creped products are also disclosed in the following patents: U.S. Pat. No. 3,994,771 to Morgan, Jr. et al.; U.S. Pat. No. 4,102,737 to Morton; and U.S. Pat. No. 4,529,480 to Trokhan. The processes described in these patents comprise, very generally, forming a web on a foraminous support, thermally pre-drying the web, applying the web to a Yankee dryer with a nip defined, in part, by an impression fabric, and creping the product from the Yankee dryer.
As noted in the above, throughdried products tend to exhibit enhanced bulk and softness; however, thermal dewatering with hot air tends to be energy intensive and requires a relatively permeable substrate. Thus, wet-press operations are preferable from an energy perspective and are more readily applied to furnishes containing recycle fiber which tends to form webs with less permeability than virgin fiber.
The state of the art is further illustrated in the following patents. It will be appreciated that high production rates (sheet speeds) are exceedingly important to the viability of many production processes. In connection with paper manufacture, it has been suggested, for example, to employ an air foil to stabilize web transfer off of a Yankee dryer in order to maintain suitable production rates. There is disclosed, for example, in U.S. Pat. No. 5,891,309 to Page et al. a foil positioned adjacent a Yankee dryer above a creping doctor. The foil is designed to stabilize the web as it leaves the dryer and includes an air deflector positioned tangent to the Yankee dryer. The web is held against the bottom side of the foil by one or more Coanda air jets which are directed over the bottom surface of the foil. The jets are intended to prevent the web from sticking to the bottom surface of the foil while creating a Bernoulli effect which holds the web against the foil. See also, U.S. Pat. No. 5,512,139, to Worcester et al. which discloses a static foil (46, FIG. 1) intended to stabilize a sheet. Another method of facilitating transfer off a can dryer is disclosed in U.S. Pat. No. 5,232,555 to Daunais et al.
U.S. Pat. No. 5,851,353 to Fiscus et al. teaches a method for can drying wet webs for tissue products wherein a partially dewatered wet web is restrained between a pair of molding fabrics. The restrained wet web is processed over a plurality of can dryers, for example, from a consistency of about 40 percent to a consistency of at least about 70 percent. The sheet molding fabrics protect the web from direct contact with the can dryers and impart an impression on the web.
U.S. Pat. No. 5,087,324 to Awofeso et al. discloses a delaminated stratified paper towel. The towel includes a dense first layer of chemical fiber blend and a second layer of a bulky anfractuous fiber blend unitary with the first layer. The first and second layers enhance the rate of absorption and water holding capacity of the paper towel. The method of forming a delaminated stratified web of paper towel material includes supplying a first furnish directly to a wire and supplying a second furnish of a bulky anfractuous fiber blend directly onto the first furnish disposed on the wire. Thereafter, a web of paper towel is creped and embossed.
U.S. Pat. No. 5,494,554 to Edwards et al. illustrates the formation of wet press tissue webs used for facial tissue, bath tissue, paper towels, or the like, produced by forming the wet tissue in layers in which the second formed layer has a consistency which is significantly less than the consistency of the first formed layer. The resulting improvement in web formation enables uniform debonding during dry creping which, in turn, provides a significant improvement in softness and a reduction in linting. Wet pressed tissues made with the process according to the '554 patent are internally debonded as measured by a high void volume index.
Other processes such as wet crepe, throughdry processes have been suggested in the art and practiced commercially. One such process is described in U.S. Pat. No. 3,432,936 to Cole et al. The process disclosed in the '936 patent includes: forming a nascent web on a forming fabric; wet pressing the web; drying the web on a Yankee dryer; creping the web off of the Yankee dryer; and through-air drying the product.
Another wet crepe, through-air dry process is suggested in U.S. Pat. No. 4,356,059 to Hostetler. In the '059 patent there is disclosed a process including: forming a nascent web on a forming fabric; drying the web on a can dryer; creping the web off of the can dryer; through-air drying the web; applying the dry web to a Yankee dryer; creping the web from the Yankee dryer and calendering the product.
Wet crepe, through-air dry processes have not met with substantial commercial success since the process rates, product quality and machine productivity simply could not meet the demanding criteria required in the industry.
It has been found in accordance with the present invention that a wet crepe process can run at high productivity and provide a range of quality products provided certain elements of the process are properly controlled. Salient features of the present invention include: (a) creping a partially dried web off a heated dryer and (b) controlling the microstructure of the wet web such that the web is suitable for transpiration or throughdrying at high rates. These features and numerous other aspects of the present invention are described in detail below.